In known vehicle speed control systems, typically referred to as cruise control systems, a set-speed for the vehicle may be initially set by manually bringing the vehicle up to the desired speed and then manipulating a user-selectable user interface device, such as, for example, a pushbutton, to set the prevailing vehicle speed as the set-speed. When the user wants to change the set-speed thereafter, the same or different user input device(s) may be manipulated to increase or decrease the set-speed. In response to a requested or commanded change in set-speed, the speed control system causes the vehicle to accelerate or decelerates as appropriate, to reach or match the new set-speed by sending commands to one or more vehicle subsystems, such as, for example, the powertrain and/or brake subsystems of the vehicle.
Conventional speed control systems are not without their drawbacks, however. For example, white the use of a speed control system designed particularly for on-highway or on-road use at low speeds when driving off-road may offer the user of a vehicle considerable advantages in reduced user workload and enhanced vehicle composure, if the user attempts to negotiate an off-road obstacle such as a boulder field, then the vehicle speed would likely either be too great (typically on-highway/on-road cruise control systems have a minimum set speed of around 30 mph (approximately 50 kph)) or the vehicle engine would likely stall during the extremes of torque requirement necessary to negotiate such an obstacle.
Similarly, while speed control systems designed particularly for use at low speeds when driving, for example, off-road, may also offer the user advantages as it relates to user workload, vehicle composure, and driver comfort, as the vehicle transitions from a environment requiring a relatively large amount of drive torque (e.g., an incline, sand, water, mud, etc.) to an environment requiring a substantially less amount of drive torque (e.g., a decline, a flat surface, pavement, etc.), the vehicle may experience powertrain or engine overrun as the elevated torque demand passes, thereby causing the vehicle speed to exceed the set-speed of the speed control system. For example, as a vehicle is negotiating an obstacle such as a boulder field using a speed control system having a particular set-speed, varying amounts of drive torque will be required to maintain the set-speed depending on, for example, whether the vehicle is ascending a boulder, travelling along the tops of one or more boulders, or descending a boulder. While a relatively large amount of torque may be required to lift the vehicle over a boulder, a much lesser amount will be required as the vehicle crests the boulder, and thus, the drive torque must be appropriately decreased to maintain the vehicle speed at the set-speed. However, due to a lag in the response of an internal combustion engine to changes in torque demand (i.e., torque output lags torque demand), as the vehicle crests the boulder, powertrain or engine overrun may occur thereby causing the speed of the vehicle to at least temporarily exceed the set-speed of the speed control system until the engine or powertrain can reduce the drive torque to an appropriate level. As a result, the driver or user of the vehicle may perceive the vehicle to be lurching over the boulder as opposed to negotiating it at a constant, smooth speed.
Accordingly, there is a need for a speed control system and method for use with the same that minimizes and/or eliminates one or more of the above-identified deficiencies.